1. Field of the Invention
This invention relates to a gas detector used with an exhaust gas purifier system of feedback type employing, for example, a three-way catalyst.
2. Description of the Prior Art
In order to reduce or eliminate the obnoxious components contained in the automobile exhaust gas, the ratio between the air taken into the engine and the fuel, i.e., what is called the air-fuel ratio is required to be regulated accurately. Especially, if the obnoxious components are to be reduced simultaneously by the use of a three-way catalyst, the air-fuel ratio is required to be regulated in a range very near to the stoichiometric air-fuel ratio. For this purpose, a detector is necessary which makes possible accurate regulation of the air-fuel ratio on the one hand and is durable on the other hand. The conventional well-known detector includes an oxygen concentration detector element made of such a material as zirconium oxide, or a gas detector element using a metal oxide for indicating changes in electric resistance dependent on the oxygen pressure component.
As compared with the oxygen concentration detector element for detecting the changes in electromotive force which acts as an oxygen concentration cell using a solid electrolyte such as zirconium oxide, the gas detector element using a metal oxide, especially, a sintered oxide of a transition metal has the advantage that electrodes are easily mounted onto the gas detector element and the versatility in the shaping thereof enables easy manufacture with a lower cost. The conventional gas detector elements, however, have a considerably longer response time to the changes in concentration of the gas components than the oxygen concentration detector element.
The response time is the sum of the time required for fresh gas to penetrate the inner part of the detector element by replacing the old gas therein, and the time before such a gas is absorbed by particles of the metal oxide or by mutual coupling portions of such particles making up the gas detector element present intermediately of a pair of pick-up electrodes in spaced relation and oxidation or reduction of the metal oxide particles is caused to increase or decrease the amount of free electrons. In the case of a gas detector element made of an oxide of an n-type semiconductor metal of TiO.sub.2, for instance, the electric resistance of the oxide is low in a reduction atmosphere where the air-fuel ratio is higher than the stoichiometric air-fuel ratio and high in an oxidation atmosphere where the air-fuel ratio is lower than the stoichiometric air-fuel ratio. Comparison between the response time when the air-fuel ratio is transferred from the higher to lower value and the response time when the air-fuel ratio is transferred from the lower to higher value shows that the response time is shorter when the air-fuel ratio is transferred from the lower to higher value. This is for the reason that when the air-fuel ratio is transferred from the lower to the higher side, the high electric resistance of the surface portion of the oxide which results on the lower side of air-fuel ratio decreases by contacting the gas of lower air-fuel ratio to cause a sudden change in the entire resistance, while when the air-fuel ratio is transferred from the higher to lower side, the entire resistance does not suddenly change until the gas penetrates inner part of the gas detector element to change the low resistance on the lower side to higher resistance. For this reason, it is important to shorten the time required for the gas to penetrate the inner part of the gas detector element.